This application claims priority of European Application No. 00201761.4, filed on May 19, 2000.
1. Field of the Invention
The invention relates to an aqueous-based cross-linkable binder composition comprising
(A) an aqueous dispersion of a polyester-polyacrylate hybrid resin 50 to 90 wt. % of which is composed of polyester and 10 to 50 wt. % of which is composed of polyacrylate, which hybrid resin is obtainable by grafting a composition of unsaturated monomers onto a partially unsaturated hydroxy-functional polyester resin, and
(B) an organic polyisocyanate, and to its use for the production of a coating, lacquer or sealing composition.
2. Discussion of the Prior Art
Two-component aqueous-based binder compositions based on hydroxyl-functional polyester-polyacrylate hybrid resins and organic polyisocyanates are known from, int. al., WO 97/19118 and WO 94/13720.
WO 97/19118 discloses a water-dissipatable polymer comprising an at least partially unsaturated polyester having a molecular weight between 300 and 3,000, an OH number between 20 and 300, and an acid number  less than 5 mg KOH/g, onto which polyester a sulphonic acid salt groups-containing acrylic polymer has been grafted. A drawback to the aqueous-based coating compositions disclosed therein is that they suffer from excessive foaming when applied via spraying.
WO 94/13720 discloses a polymer dispersion based on polyester and acrylate having a solids content of between 30 and 55% by weight which can be obtained by free-radical polymerisation of from 80 to 20% by weight of at least
(a) one (meth)acrylic acid ester of mono- or polyalcohols wherein said esters can additionally contain free hydroxyl groups; and
(b) one ethylenically unsaturated, at least monofunctional acid; and optionally
(c) an ethylenically unsaturated monomer which is distinct from (a) and (b) or mixtures thereof,
with from 20 to 80% by weight of a free-radical polymerisable polyester resin having from 0.2 to 1.5 polymerisable double bonds per molecule, obtainable by the polycondensation of at least divalent polyalcohols with at least divalent polycarboxylic acids or their derivatives in the presence of polyalcohols and/or polycarboxylic acids containing at least one polymerisable double bond.
However, if such polymer dispersions of aqueous organic polyester-acrylate polyols are combined with polyisocyanates, the presence of organic amines used for at least partially neutralising the carboxylic acids can catalyse the undesired reaction between water and isocyanate. This produces carbon dioxide, which can foam, leading to undesirable effects, such as imperfections in a coating layer. Accordingly, a drawback to the known aqueous-based coating compositions is that either their pot life is open to improvement or they cause excessive foaming during spraying.
The invention now provides aqueous-based binder compositions which do not detract from the performance due to excessive foaming and which have an acceptable pot life and cure speed, which compositions comprise aqueous polyester-polyacrylate hybrid dispersions having a solids content of more than 30% by weight at a viscosity of 0.1 to 5 Pa.s and an organic polyisocyanate.
The invention provides aqueous-based cross-linkable binder compositions comprising (A) an aqueous dispersion of a polyester-polyacrylate hybrid resin 50 to 90 wt. % of which is composed of polyester and 10 to 50 wt. % of which is composed of polyacrylate, which hybrid resin is obtainable by grafting a composition of radically polymerisable unsaturated monomers onto a partially unsaturated hydroxy-functional polyester resin, and (B) an organic polyisocyanate, characterised in that the partially unsaturated hydroxy-functional polyester has a hydroxyl value of 50 to 350, a COOH value of at most 10 mg KOH/g, and a number average molecular weight, optionally after chain extension with a polyisocyanate, of from 400 to 3,000, and is obtainable by reaction of a mixture of polycarboxylic and, optionally, monocarboxylic acids, at least 50 mole % of such poly- and/or monocarboxylic acids are aliphatic acids with 6-12 carbon atoms, with one or more (cyclo)aliphatic alcohols comprising an aliphatic and/or a cycloaliphatic diol, wherein 0.5-6 mole % of the acids and/or alcohols are ethylenically unsaturated, and, optionally, a mono- and/or bisepoxide, and/or a tri- or higher-functional polyacid and/or polyalcohol, and the composition of radically polymerisable unsaturated monomers comprises a mixture of hydrophobic and hydrophilic monomers, wherein
(a) the hydrophobic monomers are selected from the group of aromatic vinyl compounds and (cyclo)alkyl(meth)acrylates with 4 or more carbon atoms in the (cyclo)alkyl group, and
(b) the hydrophilic monomers are selected from the group of (meth)acrylic acid, (meth)acrylamide, (meth)acrylonitrile, optionally substituted, unsaturated monomers containing a sulphonic acid salt group, hydroxy-alkyl(meth)acrylates, and, optionally, unsaturated monomers containing nonionic groups such as C1-C4 alkoxy polyalkylene oxide groups, wherein the molar ratio of unsaturated carboxylic acid groups to unsaturated sulphonic acid groups is between 1:1 and 4:1, the molar ratio of components (a) to components (b) is between 1:2 and 3:1, the COOH value is between 20 and 80, and the SO3H value is between 10 and 40, and wherein the carboxylic acid groups are at least partly neutralised.